Environmental Engineering Reference
In-Depth Information
Figure
19
shows the dependence of the typical mass spectrum on the laser
frequency. It indicates that the wavelength 532 nm is better for low degree of
fragmentation though 355 nm gives highest spectrum intensity. The peak assign-
ment for the spectra in Fig.
19
is listed in Table
1
. Except for CH, OH, C
2
, and
H
2
O, all other intermediate species is dif
cult by spectroscopical methods described
above. (Our most recent challenges include visualization by infrared spectroscopy
to overcome this, which is not described here.) Most of important intermediate
species during methane/air combustion are successfully detected. These includes
the species dehydrogenated during oxidation of the fuel methane, CH
n
, such as
CH
3
,CH
2
, and CH as well as unburnt CH
4
, and C
2
H
m
formed by coupling of C
1
H
n
.
Another interesting species detected LIMS is CH
3
O, or methoxy radical which
clearly re
ects the beginning pathway of fuel oxidation.
The next step is to evaluate the quantitative property of LIMS. Figure
20
shows
the comparison of the OH pro
fl
les obtained by LIMS mapping and PLIF. These
pro
les are almost same geometrically. The intensity difference is probably due to
the
flow disturbance caused by the sampling cone interface (Fig.
18
b).
Figure
21
shows the typical pro
fl
les of CH
n
intermediate species for methane/air
premixed combustion
flame under the different equivalence ratios. Under the lean
fuel condition, all the reductive (able to be oxidized) or combustible species, CH
n
,
do not exist around the peripheral zones. This is attributable to the very fast
combustion reaction under the premixed condition.
fl
CH
4
4
4
[-]
[-]
4
3
3
3
1
1
1
11
-
0
-
0
-
0
0.9
-2
-2
-2
-
-3
-
-3
-
-3
0.8
0
5
10
15
20
25
30
0
5
10
15
20
25
30
0.7
0
5
10
15
20
25
30
z [mm]
z [mm]
z [mm]
0.6
0.5
CH
4
4
4
3
3
3
3
0.4
2
1
1
0.3
-
0
0
-
0
-
-1
0.2
-2
-2
-
-3
-3
-
-3
0.1
-4
0
5
10
15
20
25
30
0
5
10
15
20
25
30
0
5
10
15
20
25
30
0
z [mm]
z [mm]
z [mm]
CH
4
4
4
2
3
3
2
2
1
1
-
0
0
-1
-
-1
-2
-
-2
-
-3
-3
-4
-4
0
5
10
15
20
25
30
0
5
10
15
20
25
30
0
5
10
15
20
25
30
z [mm]
z [mm]
z [mm]
CH
4
4
4
3
2
2
2
1
1
0
-
0
-1
-
-1
-2
-
-2
-3
-
-3
-4
-4
0
5
10
15
20
25
30
0
5
10
15
20
25
30
0
5
10
15
20
25
30
z [mm]
z [mm]
z [mm]
1.3
0. 9
1.0
Fig. 21 LIMS profiles of CH
n
species for methane/air premixed combustion
fl
ame under the
different equivalence ratios
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